1. Field of the Invention
This invention relates to a dry etching method employed for the preparation of semiconductor devices. More particularly, it relates to a dry etching method for performing high aspect ratio processing, such as silicon trench etching, with satisfactory anisotropy.
2. Description of Related Art
In keeping up with the high integration and high performance of semiconductor devices, such as VLSIs or ULSIs, the aspect ratio of various patterns in the semiconductor devices tends to be increased acutely. Thus, in the field of dry etching, there is a keen demand for a technology of performing high aspect ratio processing with satisfactory anisotropy.
A typical example of high aspect ratio processing of the layer of the silicon based material is silicon trench etching aimed at forming capacitative elements or device isolation. Trench depth varies depending on the device type and usage and is of the order of 4 to 5 .mu.m, 1 .mu.m and 4 .mu.m for capacitative elements, device isolation for MOS transistors, and device isolation for bipolar transistors, respectively. The aperture diameter is of the order of 0.35 to 1.0 .mu.m in each case. With silicon trench etching, the cross-sectional shape of the trench is changed in a complex manner depending on mask patterns or etching parameters to present difficulties in trench filling or capacitance control to be performed in subsequent steps. Therefore, if fluorine based gases, with which etching proceeds mainly on the basis of radical reactions, are used for etching, difficulties are raised in achieving high anisotropy. For this reason, chlorine based gases, as typified by a gas mixture of SiCl.sub.4 and N.sub.2, are employed. With this gas mixture, Cl* are generated as a main etchant from SiCl.sub.4 while N.sub.2 is used for deposition of Si.sub.x N.sub.y Cl.sub.z which is used for sidewall protection for achieving high anisotopy. The present Assignee has previously proposed adding fluorine based gases capable of generating F*, such as ClF.sub.3, to the above gas system, in an amount which is not obstructive to anisotopy, with a view to elevating the etchrate which is determined by the reaction between Cl* and single crystal silicon.
Meanwhile, low pollution is also demanded of dry etching, besides high anisotropy and high etchrate, as mentioned previously. Above all, in silicon trench etching, for which prolonged processing time is required, how to suppress particle generation during etching is crucial. Thus, for reducing pollution, it is desirable to diminish the generation of reaction products, such as Si.sub.x N.sub.y Cl.sub.z. However, in this case, anisotropy would be deteriorated because the effects of the fluorine based gases, added for increasing the etchrate, would be demonstrated pronouncedly.
The present Assignee has also proposed a technique of etching a layer of a silicon based material using an etching gas consisting mainly of sulfur chloride, such as S.sub.2 Cl.sub.2, or sulfur bromide, such as S.sub.2 Br.sub.2, as a gas system effective to suppress pollution by the particles. With this technique, anisotropy may be attained through sidewall protection and suppression of radical reaction which may be achieved by low temperature etching. If, for example, S.sub.2 Br.sub.2, is used, sidewall protection may be attained by a deposit mainly composed of sulfur dissociated from S.sub.2 Br.sub.2 and also containing SiBr.sub.x which is an etching reaction product. This deposit may easily be removed by sublimation or vaporization by heating the etched substrate to approximately 90.degree. to 150.degree. C. after completion of etching, so that there is no risk of pollution by the particles. Despite such an advantage, the etchrate is generally low with the prior art technique because the rate of the etching reaction itself is determined by the ion assist reaction by Cl.sup.+ or Br.sup.+.
Thus a demand has been raised for a more excellent process in view of the difficulties encountered in simultaneously satisfying the requirements for high anisotropy, high etchrate and low pollution.